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Chemistry in its element: rhenium

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You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

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Meera Senthilingam

This week one of the rarest elements on earth that seems to enjoy changing the laws of nature. Unraveling the mysteries of rhenium, here's UCLA's Eric Scerri.

Eric Scerri

Rhenium is element 75 in the periodic table and in many ways a rather unusual element. It is one of the rarest elements on the Earth with an abundance of something like 1 part per million. It is also one of the densest elements, following only platinum, iridium and osmium and it is one of the highest melting point elements exceeded only by tungsten and carbon.

Rhenium sits two places below manganese in the periodic table and its existence was first predicted by Mendeleev when he first proposed his periodic table in 1869. In fact this group is unusual in that, when the periodic table was first published, it possessed only one known element, manganese, with at least two gaps below it. The first gap was eventually filled by element 43 technetium, the second gap was filled by rhenium. But rhenium was the first to be discovered.

It was first isolated in 1925 by Walter and Ida Noddack and Otto Berg in Germany. In the course of an extraction of epic proportions, they processed about 660 kg of the ore molybdenite in order to get just one gram of rhenium. These days rhenium is extracted more efficiently as the bi-product of the processes for the purification of molybdenum and copper, since rhenium often occurs as an impurity in the ores of these elements.

The discoverers called their element, rhenium, after the Latin name Rhenus for the river Rhine close to the place where they were working. In fact the Noddacks and Berg believed that they had also isolated the other element missing from group 7, or element 43, that eventually became known as technetium, but it was not to be.

As recently as the early years of the 21st century some researchers from Belgium and the US re-analyzed the X-ray evidence from the Noddacks and argued that they had in fact isolated element 43. But these claims have been hotly debated by many radiochemists and physicists and now have been finally laid to rest.

But by an odd twist of fate, a Japanese chemist, Masataka Ogawa believed that he had isolated element 43 and called it nipponium back in 1908. His claim too was largely discredited but as recently as 2004 it has emerged that he had in fact isolated rhenium well before the Noddacks.

Until quite recently no mineral containing rhenium combined with just a non-metal had ever been found. Not until 1992 that is, when a team of Russian scientists discovered rhenium disulphide at the mouth of a volcano on an islands off the east coast of Russia between the Kamchatka peninsula and the Japanese islands.

The chemistry of rhenium is also rather interesting. For example, it shows the largest range of oxidation states of absolutely any known element, namely -1, 0, +1, +2 and so on all the way to +7, the last of which is actually its most common oxidation state.

Now here is another oddity. Until the early 1960s it was believed that three bonds between any two atoms was as high as nature could go, as in the case of the nitrogen-nitrogen triple bond for example. But in 1964 Albert Cotton and co-workers in the USA discovered the existence of a metal-metal quadruple bond. Yes you guessed it, it was rhenium, or rather a rhenium compound namely the rhenium ion, [Re2Cl8]2+ [correction: this should be the two minus ion, not the two plus ion].

More recently an especially simple compound of rhenium, rhenium dibromide, has attracted a great deal of scientific attention because it is one of the hardest of all known substances. And unlike other super-hard materials, like diamond, it does not have to be manufactured under high pressure conditions.

But what else is rhenium good for? What are some other applications? Well there are many of them. A good deal of the rhenium extracted is made into super-alloys to be used for parts in jet engines. Not surprisingly for a transition metal, rhenium is also a good catalyst. In fact a combination of rhenium and platinum make up the catalyst of choice in the very important process of making lead-free and high-octane petrol. Rhenium catalysts are especially resistant to chemical attack from nitrogen, phosphorus, and sulfur, which also makes them useful in hydrogenation reactions in various industrial processes.

And just to go back to the Noddacks, and in particular Ida Noddack, it was she who first proposed in 1934 that nuclear fission might be possible as the result of the break up of a nucleus into fragments but her speculation was ignored and it had to wait until 1939 when Hahn, Strassmann and Meitner really discovered fission. Why was Noddack ignored? The most popular view seems to be that it was because her reputation had been damaged by her falsely announcing the discovery of element 43 in addition to the correct discovery of rhenium.

Meera Senthilingam

So its one of the hardest of all known substances, has a variety of oxidation states, and has the ability to make quadruple bonds, certainly a rule breaker. That was Eric Scerri from the University of California Los Angeles, revealing the secret powers of rhenium. Next week, a colourful luminous element.

Louise Natrajan

Terbium in the +3 state radiates an aesthetically pleasing luminous green colour when the correct wavelength of energy is used to excite the atoms. This is because terbium 3+ ions are strongly luminescent, so strong in fact, that its luminescence can often be seen by the naked eye The human eye is particularly sensitive to the colour green and even small amounts in the right compound are easily detectable by eye. This bright colour renders terbium compounds particularly useful as colour phosphors in lighting applications, e.g. in fluorescent lamps, where it is a yellow colour, and as with europium(III) which is red, provides one of the primary colours in TV screens; who knew that Turbium could be in your TV set!

Meera Senthilingam

And Manchester University's Louise Natrajan will be filling us in on the colourful story of terbium in next week's Chemistry in its element. Until then I'm Meera Senthilingam and thank you for listening.